Many vehicle engine control systems regulate fueling to the engine in response to measured or estimated air flow into the engine so that the air and fuel mixture into the engine provide a stoichiometric mixture. It is typical for a vehicle to include an exhaust oxygen sensor in the exhaust flow path to sense whether a fuel-rich condition (i.e., excess hydrocarbons and carbon monoxide) or a fuel-lean condition (i.e., excess oxygen) exists in the exhaust. The engine controller responds to the oxygen sensor by adjusting the air fuel ratio entering the engine to minimize the rich or lean excursions of the exhaust gas, ideally maintaining the average exhaust gas composition at stoichiometry.
It is known practice to run the engine in a fuel rich condition at start up to (a) aid in the operation of the cold engine and (b) provide fuel for reaction in the exhaust path downstream of the engine to quickly heat up the vehicle's catalytic converter. For example, an air pump is placed to pump atmospheric air in the exhaust path downstream of the engine to react with fuel in the fuel rich exhaust during the time period immediately following vehicle start up. The reaction of the fuel rich exhaust and the pumped-in air, referred to as secondary or supplemental air, is exothermic and serves to heat up the catalytic converter. Various approaches have been taken for providing the secondary air, including injecting the secondary air directly into the exhaust ports of the engine so that the hot exhaust exiting the combustion chambers ignites with the injected air, or injecting the air closer to the catalytic converter and using a supplemental heat source such as an electric heater to ignite the air and fuel rich exhaust. Even further alternatives include injecting air between two catalyst-coated substrates in a catalytic converter, i.e., where the upstream catalyst-coated substrate includes a reduction catalyst and the downstream catalyst-coated substrate includes an oxidizing catalyst. Because of the nature of air pumps and their control, open loop flow control of secondary air is often not precise, limiting the capable accuracy of the air fuel ratio control in the exhaust system downstream of the engine.
Mass air flow sensors may be placed within the pump's air path to provide a direct measurement of secondary air flow. However, such mass air flow sensors are an expensive addition to the vehicle.